1. Field of the Invention
The present invention relates to an optical transmission circuit which creates an optical signal having a low ratio of degradation in transmission quality, caused by group velocity dispersion (GVD) of the transmission medium such as optical fiber.
2. Description of Related Art
While an optical transmission system can handle a large capacity with relative ease by the use of wavelength-division multiplexing (WDM), research is also being widely carried out to increase the speed of the bit-rate per wavelength. The reason for this is that increasing the speed of the bit-rate per wavelength reduces the cost of the apparatus, and reduces the initial cost of the total system and its running cost by miniaturizing the apparatus and reducing its power consumption. An electrical circuit which realizes channel power of 40-Gbit/s is already in the practical stage (Reference Document 1: M. Yoneyama et al., “A 40-Gbit/s Optical Repeater Circuit using InA1As/InGaAs HEMT Digital IC Chip Set”, IEEE MTT-S Digest, WE1 D-2, 1997).
Problems in increasing the bit rate per wavelength include restrictions on the possible transmission distance by group velocity dispersion (GVD), restrictions on the input power to the optical fiber resulting from the nonlinear characteristic of optical fiber, and such like. Application of dispersion-compensating technology is effective in removing restrictions on the possible transmission distance. As for input power restrictions, RZ (return-to-zero) format has greater tolerance than NRZ (non-return-to-zero) format, which is conventionally used in optical transmission systems, and most reports of 40-Gbit/s single channel transmission experiments describe systems using RZ formats. CS (carrier-suppressed)-RZ format using an alternating phase-inverted pulse (Reference Document 2: Y. Miyamoto et al., OAA '99, PDP4-1), and DCS (duo-binary carrier suppressed)-RZ format (Reference Document 3: Y. Miyamoto et al., Dig. OFC'01, TuU4) are regarded as especially promising, since these RZ formats have comparatively lenient restrictions on the input power to optical fiber resulting from the nonlinear characteristic of the optical fiber, and on the possible transmission distance by group velocity dispersion.
One conventional method of creating CS-RZ and DCS-RZ formats uses a Mach-Zehnder optical modulator, and is comparatively easy to realize (see Reference Documents 2 and 3). According to the method which uses a Mach-Zehnder optical modulator, an alternating phase-inverted pulse is created by driving the optical modulator with a sine wave of half the frequency of the repetition frequency of the alternating phase-inverted pulse. The voltage applied to the optical modulator at this time is twice the half-wavelength voltage Vπ, which is the voltage required to maximize the on-off ratio of the optical modulator. The voltage value at which this sine wave voltage and the transmission loss of the optical modulator reach their maximums must be applied as a DC bias voltage.
However, since the Mach-Zehnder optical modulator utilizes light interference, the applied voltage characteristics of its output optical power (input-output characteristics) are known to fluctuate (Reference Document 4: Jumonji et al., Institute of Electronics, Information, and Communication Engineers, memoir C-1, J75-C-1, pp.17–26, 1992). In particular, “DC drift” is the term given to shifts in the input-output characteristics when a voltage is applied, and poses a significant problem in the practical use of the Mach-Zehnder optical modulator, which is manufactured from Z-cut LiNbO3.
A circuit which detects fluctuation in the input-output characteristics, and feeds back the voltage applied to the optical modulator has been proposed and is in practical usage; this circuit is termed a bias voltage control circuit, since the DC component is usually cut from the data signal before applying it to the MZ-modulator (Reference Document 5: Japanese Patent No. 2642499, Reference Document 6: Japanese Patent No. 2866901, Japanese Patent No. 2869585, Reference Document 7: Japanese Patent Application, First Publication No. Hei 10-24874).
In “Optical Transmitter, Control Circuit for Optical Modulator and Optical Modulating Method” in Reference Document 5, a low-frequency signal is superimposed onto a data signal, drifts in the input-output characteristics are detected based on the level of the low-frequency signal, obtained by receiving part of the output of the optical modulator, and the direction of the drift is detected based on the phase of the low-frequency signal.
However, the method of Reference Document 5 assumes that the optical modulator is driven at a voltage of Vπ, the low-frequency signal component being cancelled during modulation at 2Vπ; consequently, it is not possible to detect error signals or control the bias voltage.
“Optical Modulator Control Circuit” of Reference Document 7 differs from Reference Document 5 in that the low frequency signal is superimposed onto the bias voltage instead of a data signal; the detection of drift in input/output characteristics is substantially the same as in Reference Document 5.
However, in the method of Reference Document 7, the modulated signal is subjected to envelope detection, and therefore, the photoelectric converter, DC low frequency removing circuit, and the envelope curve detector circuit all require a band which is equal to or greater than the repetition frequency of the alternating phase-inverted pulse light. Therefore, considering application in a high-speed transmission system of 20 Gbit/s or more, in which, for example, CS-RZ and DCS-RZ formats are valid, the control circuit would be extremely expensive.
In “Optical Modulator Device” of Reference Document 6, in order to detect drift, a probe light different from the main signal light is input from the opposite direction into an optical modulator having a traveling-wave electrode (most high-speed Mach-Zehnder optical modulators are of this type). Drift is detected by utilizing the fact that light input from the opposite direction is not modulated.
However, Reference Document 6 requires a light source for detection, making this constitution comparatively expensive; in addition, in creating the alternating phase-inverted pulse light, the level of the probe light decreases in order to achieve a bias voltage which will minimize the output of the optical modulator, making detection difficult.
Furthermore, in a WDM transmission system, the probe light may cause noise for other channels, requiring sufficient care in selecting a wavelength for the probe light.
The object of the present invention is to provide an optical transmission circuit comprising an inexpensive bias control circuit which causes as little deterioration as possible in the main signal light, that is, the alternating phase-inverted pulse light.